Hepatocytes and most other cells show a somewhat stereotyped pattern of cellular and organelle response when dying from acute lethal injury regardless of the inciting agent or event, e.g., chemical injury or acute anoxia. Recent evidence from several laboratories is consistent with the view that the injury reflects an alteration in the control of intra- cellular calcium homeostasis. Thus, fundamental information about the nature of chemical-induced structural alterations in plasma and endoplasmic reticulum membranes, which control calcium homeostasis, is critical to an illumination of the relevant functional consequences of such damage. Identification of the membrane alterations produced by chemical-induced lethal injury would permit an assessment of potential reversibility prior to the "point-of- no-return" stage with cellular accumulation of large amounts of calcium, thereby potentially opening new therapeutic avenues to the treatment of drug lesions and perhaps other disease states, such as treatments aimed at reducing cell swelling and ion fluxes while still at the reversible stage of cell injury. It much be emphasized, however, that these current concepts about the pathogenesis of lethal cell injury have come mainly from cell culture studies of toxic conditions other than the lethal injury caused by chemically reactive drug metabolites. Thus, direct extrapolation of the concepts to drug-induced lethal injury has been uncertain. The current work, however, in demonstrating very significant losses of plasma membrane and/or endoplasmic reticulum Ca2+ pump activities after hepatotoxic doses of acetaminophen, CCl bromobenzene, and diquat in vivo has identified what may represent the fundamental link between hypotheses of toxicity via alkylation or peroxidation and the notion of cell injury through altered Ca2+ homeostasis. Future studies proposed are: 1) establish the correlation between dose dependence of toxicity and lesions at Ca regulatory sites, 2) to investigate the role of changes in membrane Ca permeability (observed with CCl and diquat) and Ca2+ -ATPase activity in membrane lesions, and 3) to document that alterations in cellular Ca2+ homeostasis correlate directly with the membrane functional deficits observed. The overall aim of this project is to test the hypothesis that an alteration in Ca homeostasis resulting from toxin-induced membrane lesions is the final common pathway for toxic lethal injury, and further, that these lesions are produced by alkylation and peroxidation mechanisms, and not by protein-thiol oxidation.